Products constructed using conventional technology are generally built using a "top-down" approach. Top-down refers to the current way of fabricating most of today's products, using large and expensive machines to manipulate matter in bulk. While miniaturization of devices using top-down technology has increased performance and efficiency, the use of top-down technology to miniaturize devices becomes increasingly difficult and expensive with the decrease in the size of the fabricated object. For instance, conventional techniques for etching circuit patterns, particularly in microcircuits, it is difficult to carry out stable and uniform etching methods when the printed circuits have a width of 0.1 mm or less.
An alternative to top-down technology, a so-called "bottom-up" approach, refers to the fabrication of objects from a set of small, fundamental building blocks, which cannot be reduced further. Complex objects are fabricated by creating and assembling these building blocks using a specified sequence of construction steps. This technique is very similar to creating software, where the building blocks of information (bits) are arranged in useful patterns.
Molecular assembly presents a `bottom-up` approach to the fabrication of objects specified with incredible precision. Molecular assembly includes construction of objects using tiny assembly components, which can be arranged using techniques such as microscopy, e.g. scanning electron microspray. Microelectrodeposition and microetching can also be used in microfabrication of objects having distinct, patterned surfaces.
Molecular self-assembly is the spontaneous association of molecules under equilibrium conditions into stable, structurally well-defined aggregates joined by noncovalent bonds. Molecular self-assembly is ubiquitous in biological systems and underlies the formation of a wide variety of complex biological structures. Self-assembly is also emerging as a new strategy in chemical synthesis, with the potential of generating nonbiological structures with dimensions as small as 1 to 100 nanometers, and having molecular weights of 10.sup.4 to 10.sup.10 daltons. Structures even in the upper portion of this range of sizes are presently difficult to attain through chemical synthesis, and the ability to prepare them would open a route to structures comparable in size (and perhaps complementary in function) to those that can be prepared by microlithography and other techniques of microfabrication. G M Whitesides et al., Science 254:1312-9 (1991).
Regular arrays of topologically complex, millimeter-scale objects can also be prepared by self-assembly, with the shapes of the assembling objects and the wettability of their surfaces determining the structure of the arrays. N. Bowden et al., Science 276:233-5 (1997). DNA molecular structures and intermolecular interactions are particularly amenable to the design and synthesis of complex molecular objects, and it has been shown that two-dimensional crystalline forms of DNA can self-assemble from synthetic DNA double-crossover molecules. E Winfree et al., Nature, 394539-44 (1998).
There is a need in the art for a systematic and reproducible method of providing structural components for the fabrication of small objects. There is also a need in the art for a method of modifying very small objects by the directed placement of particles.